For building ships the absolutely easiest thing is to look at the various threads in the the "Bureau of Ship Design" section of the forum. There you should be able to find many examples of ship designs and their general uses. Why not post a few of your own designs and people will tell you your most glaring problem or flaws so you can correct them.
When it comes to building sensors for your ships there are probably as many answers as thee are players playing this game since their designs depends on your play stile as much as efficiency. Personally I like to economise all my designs as much as I can, much depending on the current game I play. For me this means that I try to use many small sensors and make extensive use of smaller scout crafts. The benefit of this is that you can get away with building smaller sensors that are quite easy to research. On a 250 ton fighter scout you can usually fit a size three sensor while a 500 ton fighter I usually fit a size 5 sensor and a size 1.5 EM sensor.
Also remember that it is easier for the enemy to pick up your EM signature the higher the resolution on your scanners you use. As an example, if you use a resolution 100 scanner then a size one EM sensor of the same tech level as you will detect the EM signature at the same time you detect a 5000 ton object and bigger. While if you use a resolution 50 scanner you can detect a size 2500 ton object and the enemy would need an a passive EM of size two using the same tech level to detect you at the same range. Therefore it can be beneficial to use very low resolutions on smaller scout crafts, but it also mean they have to get closer to scan their intended objects.
I would say that resolution 20 scanner on scout crafts are generally a good idea. Anything from 60-120 resolution on regular ships are what you want. Just pick one and try to stick with the choice to standardize or pick one based on the size of your enemies ships when you find them.
Bigger resolutions scanner will turn your ships into EM magnets but they can be useful anyway. If you want to be aggressive then bigger resolutions means bigger active range and if you know that the enemy has larger ships that you can detect then why not. Put them on ships that you only turn on the sensor once you are detected anyway.
My regular sizes for scanner are 1,5,20,80 and 240. But I usually redesign them once I meet an enemy and tailer them after their ships.
You also need to remember to the put the right resolution on your MFC "Missile Fire Controls". In general it is easiest to have an active sensor and an MFC that uses the same resolution and distance, not necessarily on the same ship but in the same task-group at least. There are, in general, only reason to include more than one sensor in a group for backup if one of them is damaged if the ship carrying it is destroyed.
When it comes to designing the ships themselves you need to think in terms of efficiency versus versatility and economy overall. Do you have plenty of fuel reserves, if so, do you want high powered engines (with a higher risk of exploding) or do you need fuel efficient drive to conserve your overall fuel reserves?
What economy do you have in building naval yards?
In general I have found out that it is cheaper to actually build bigger ships in the long run. They are cheaper to (in both minerals and time) refit after built and they build much faster per tonnage as well and so they will keep up with your tech progression much faster than smaller ships. The reason they are cheaper is that they need much less retooling done since they usually change much less tonnage per retooling between versions.
The drawback with big ships though are that they present much bigger thermal images to an enemy and so can be hard to sneak up on enemies with. Smaller escort ships can also be good if they have tonnage that are less than the normal resolution of your enemy. If an enemy run with resolution 100 active scanners for the most part then building escorts in sizes of 4000 tons means that they most often will target your bigger ships first and leave your frail escorts alone to do their job.
Lets say that you want to build a task-group of destroyers that should be self sufficient and able to both attack and defend adequately. Let's say they will be standardised around 9000 tons and you will produce five ships for each task-group. You would like to have one designated as the leader, this ship could be fitted with a jump engine and carry most of the battle groups sensors. Although, you should keep some backups on your other ships as well, smaller but sufficient to at least make the group survive if the leader is lost.
You then either design one or two other ships. You either go with the principle that every ship should do one thing and do it well or make them dual purpose and keep your production much simpler. If you go with the first choice you might build one destroyer that primarily mount anti-ship missiles and launchers and one ship that primarily carry anti-missile launchers and gauss cannons for point defence.
I hope I have given you some ideas to try and experiment with.
